American civil liberties campaigners have welcomed a Supreme Court decision to uphold a ban on a law designed to protect children from porn on the internet.

The Child Online Protection Act (COPA) made it a federal crime to use the web to communicate commercial material considered harmful to minors.

But it has never been enforced following a legal challenge from the American Civil Liberties Union (ACLU).

Justice Clarence Thomas acknowledged that there were constitutional problems with the law, saying the government still remained barred from enforcing the legislation.

Instead, he sent the legal battle between free-speech advocates and those who want tighter regulation of internet porn back to a lower court in Philadelphia.

It will decide whether the law was unconstitutionally vague, too broad for other reasons or failed to survive strict scrutiny.

Free speech

"The court clearly had enough doubts about this broad censorship law to leave in place the ban, which is an enormous relief to our clients," said Litigation Director of the ACLU, Ann Beeson, in a statement.

The court has struck down other laws that attempt to reduce the adult population to reading only what is fit for children, we are confident that the court will ultimately strike down this law

Ann Beeson, ACLU

The ACLU claimed the vague nature of COPA threatened free speech and contravened the First Amendment.

Under the law, material defined as harmful to minors is decided by applying "contemporary community standards" - in other words, what an average citizen would deem offensive.

Sexual advice columns, discussion boards on gynaecology and the Philadelphia Gay News would all have been banned under the law, said Ms Beeson.

"The case is still very much a work in progress but just as the court has struck down other laws that attempt to reduce the adult population to reading only what is fit for children, we are confident that the court will ultimately strike down this law," she said.

Earlier attempts to regulate internet content in the US have failed.

The Communications Decency Act was thrown out in a landmark ruling in 1997 because of the impossibility of distinguishing between adults and children online.

The Supreme Court also voted last month against banning virtual child pornography, which uses young adults or computer-generated images to depict children in sexual explicit acts.